Frequency selective calling system



D. E. NOBLE FREQUENCY SELECTIVE CALLING SYSTEM April 3, 1951 Filed July 25, 1948 2 Sheets-Sheet l INVENTOR. Daniel E. Noble April 3, 1951 D. E. NOBLE 2,547,025

FREQUENCY SELECTIVE CALLING SYSTEM Filed July 23, 1948 2 Sheets-Sheet 2 v 52' LL llllll VI'I'I INVENTOR.

Daniel E. Noble Patented Apr. 3, 1951 FREQUENCY SELECTIVE CALLING SYSTEM Daniel E. Noble, Elmhurst, 111., assignor to M- torola, Inc., Chicago, 111., a corporation of Illinois Application July 23, 1948, Serial No. 40,330

13 Claims.

This invention relates generally to selective frequency responsive systems and more particularly to calling systems suitable for use with mobile radio communication equipment.

In radio communication networks in which one or more fixed stations and a plurality of remote mobile units intercommunicate, for satisfactory communication it is necessary that a signalling system be provided for calling the various stations similar to signalling systems used in telephone networks. Such signalling may be provided by operation of audible or visible signals at the called station to indicate to the operator that someone desires to communicate with him. In systems where operators are at all times available to receive a message, the calling system may automatically condition the receiver of the party to be contacted so that the message will be translated by his receiver only.

Although telephone calling equipment is availabie and signalling systems have been provided for other purposes, such available equipment is not of such rugged and simple construction as to be particularly adapted for vehicular installations. To meet the requirements for mobile use, selective Calling equipment must be fast in operation, should preferably be capable of operation by signals transmitted over the normal voice channel, and should not respond to voice signals or to noise which is received from the voice channel.

A system designed to accomplish this purpose is disclosed and claimed in application Serial No. 749,952, filed May 23, 1947, by Ralph J. Lense and Alfred S. l-iolzinger. The present invention is an improvement on the system disclosed in said patent application and is directed toward systems in which a plurality of selective responsive units are arranged in cascade to provide a large number of difierent calls so that a large number of individual stations can be individually selected.

An object of this invention is to provide an improved selective calling system in which a large number of difierent calls are available and in which accidental operation of the calling system, as by noise, is minimized.

A further object of this invention is to provide a selective calling system including a plurality of sections in which noise is ineffective to cause operation of any section until the preceding section has been operated.

A feature of the present invention is the provision of a selective calling system including a plurality of sections each of which is responsive to a calling signal including particular frequencies.

A further feature of this invention is the provision of a selective calling system including a plurality of sections arranged in cascade with each section being adapted to provide a control voltage when signals of the proper frequency are applied thereto only when the preceding 7 section is actuated.

A still further feature of this invention is the provision of a selective responsive system including a plurality of sections each being adapted, when a signal including the proper frequencies is applied thereto, to connect the next section to th calling signals and to provide operatin voltage therefor.

Further objects, features and advantages will be apparent from a consideration of the followil'ig description taken in connection with the accompanying drawings in which:

Fig. 1 is a circuit diagram illustrating one embodiment of this invention; and

Figs. 2, 3 and 4 illustrate other embodiments of the invention which operate on the same principle.

In practicing the invention there is provided a selective responsive device comprising a first section including a pair of electromechanical ccntactors and a circuit for charging a condenser when a signal including the proper frequencies is applied to said electromechanical contactors. The voltage across the condenser or a separate voltage source which is triggered by the voltage across the condenser is then used to energize a second circuit including additional electromechanical contactors and a second condenser. The second condenser is charged when a signal including the proper frequencies is applied to the electromechanical contactors of the second circuit. The voltage across the second condenser may be used as a control voltage for operating a signal or for providing switching to condition a receiver for operation. The control voltage will not be produced until a signal including the frequencies of all the electromechanical contactors is applied to the system and, therefore, a very large number of different calls is provided by the system. In an alternative embodiment of the invention not only is the energizing voltage transferred from one section of the system to the next but also the calling signal is applied successively to the sections by a switching means which disconnects the calling signal from one section of the system when this section 3 has responded and applies the calling signal to the next section.

Referring now to the drawings, in Fig. 1 there is illustrated a selective responsive system having input terminals IE? and ii adapted to be connected to a signal source which includes signals containing the intelligence to be received and also calling signals. The signal source may, for example, be a radio receiver or a wire line. The system includes a plurality of electromechanical contactors l2, 53, I l and I5 each having an energizing winding, the windings being designated by numerals i5, ii, iii and i9,

respectively. The windings 15, ll, l8 and I9 are connected in a series parallel arrangement with one terminal connected to the input terminal ill and the other terminal connected through contacts iii and El of relay 22 to the input terminal ii. The electromechanical contactors may be of any construction wherein intermittent contact is provided when signals of predetermined frequencies are applied to the windings thereof. The contactors may be of the vibrating reed type as disclosed in the co pending application of Marion R. Winkler, Serial No. 342, filed January 2, 1948.

The electromechanical contactors i2 and 53 will be selected so that the frequencies of operation are different but within the same frequency range. The two contactors will, therefor, intermittently close their contacts when a signal including the proper frequencies is applied to the windings l6 and Il thereof. t is to be noted that a series circuit is provided through the contactors l2 and i3, resistor 25, condenser 26 and resistor ill in parallel, and battery 2 A condenser 29 is bridged across the contactor l3. As described in the patent application Serial No. 749,952 referred to above, operation of the contactors i2 and It will cause the condenser 26 to be charged to a potential approaching that of the battery 28. This results as when contactor i2 is momentarily closed the battery 23 provides a charge on both condensers 25 and 29. Closingof the contactor i3 discharges condenser 29 but the charge remains on condenser 26 and is built up by increments each time the contactor l2 closes. By providing a bridging condenser across the contactor 52, a charge will be built up on condenser 25 each time either contactor is closed. Such an arrangement is shown in Figs. 3 and 4. g The resistor limits the charging of condenser 28 and prevents sudden charging thereof when contactors l2 and [3 are closed simultaneously. As the contactors l2 and I3 have diilerent frequencies, they will be closed simultaneously only at the beat frequency of the two contactors, and by providing contactors which operate at relatively close frequencies, the simultaneous closing will be rela tively infrequent. The resistor 21 bridged across condenser 26 is effective to discharge the condenser and iso'i such a value that the condenser will be discharged at a rate to prevent a substantial charge thereon caused by random closing of the contactors l2 and L3 as by noise. The resistor'2l also discharges the condenser 2%} after the calling signal has ceased. The resistor 2! is sufficiently large, however, so that the con- "denser 26 will charge very rapidly when the proper frequencies are continuously applied to the contactors.

As previusly stated, the windings it and !9 of contactors Hi and 15 are also connected to the signal source. When the condenser '26 is denser charged, it forms a source of potential for the second section of the selective responsive system. The second section is substantially like the first being a series circuit including contactors Hi and i5, resistor 3i}, and condenser 3| and resistor 32 in parallel. A condenser 33 is bridged across the contactor Hi. It will be apparent that this series circuit is similar to the series circuit through contactors l2 and it with condenser 26 being equivalent to the battery 28. The condenser Sl will, therefore, be charged when signals of the proper frequency are applied to the windings l8 and 59, with the voltage across the condenser 3i approaching that of the battery 28. Ihe voltage across the condenser Si is applied to the input circuit of vacuum tube 35 being connected to the grid 35 thereof and through the winding 38 of relay 22 to the cathode 3?. The plate 39 of the tube 35 is connected through resistor 40 to a source of potential. The plate resistor may not be required in certain applications. When the potential applied on the grid 36 of the tube due to the charge of condenser SI reaches a predetermined value, the tube 35 will conduct energizing the relay 22 so that the movable contact 2i is moved into contact with the fixed contact fil. This connects the loudspeaker 32 to the terminal H and, .as the speaker is permanently connected to the terminal lil, the speaker will be directly connected to the signal source. The relay will remain actuated for a short time after the calling signals cease as long as the voltage across confil remains above the predetermined value. The condenser 3i will, of course, be discharged by resistor 32 with the rate or" discharge depending on the value of the resistor. It is apparent that such'a system would be applicable for use in a radio receiver to connect the loudspeaker to the signal channel when a calling signal including a plurality of predetermined frequencies is received by said signal channel.

The two-section system as disclosed in Fig. 1 in addition to increasing the number of calls of the system, provides increased discrimination against noise. This results as energizing voltage for the second section is not available until the first has been operated. That is, until 'a voltage is built up on condenser 26, even though the noise includes frequencies which would operate contactors it and i5, these contactors would not be effective to charge the condenser 3|. If the noise included all of the frequencies required for the contactors i2, i3, M and i5, the possibilities are that the frequencies for contactors l2 and 53 would not be suificiently continuous to provide the full voltage on condenser 26 so that the effect of noise on contactors M and iii would be diminished and the voltage on condenser ill would not be great enough to actuate the relay 22. This i especially true as resistors 2i and 32 are eiiective to reduce the charge on the condensers 26 and Bi caused by random closing of the contactors. The resistors 2'5 and 32 also gradually discharge the condensers 26 'and 3l when fully charged so that for operation of the system the calling signals must follow each other in relatively short order. Also the system will remain in actuated condition for only a limited time after the calling signals cease.

Referring now to Fig. 2, there is illustrated a modified system in which the calling signals are selectively applied to the electromechanical contactors in the two'sections of the system. In this modification the contactors 58 and 5| in the first operate to charge condenser l3.

section include windings 52 and 53 connected in series across the signal source through contacts 54 and of the relay 55. When the calling signal includes the frequencies to which the contactors 59 and SI respond, the condenser 51 is charged through resistor 58 in the manner previously described. Condenser 58 is bridged across contactor 5i and a resistor provides a discharging path for the condenser 51. The winding SI of relay 56 provides energizing potential for charging the condenser. The voltage built up on condenser E? is applied to the triode 62 being connected to the grid 63 thereof and through the winding E! to the cathode B4. The plate 55 of the triode is connected through resistor 66 to a source of positive potential so that when a voltage of predetermined value is applied to the grid 53 thereof the triode d2 will conduct causing a large current flow through winding Iii of the relay. This causes the movable contact 55 of the relay to move away from fixed contact 54 and into engagement with the fixed contact 61. It will be apparent from the circuit that this operation of the relay 56 disconnects the windings 52 and 53 of contactors 50 and 55 from the signal source and connects the windings 69 and it! of contactors ii and 12 thereto.

After the first section of the system has thus been operated, in the event that the calling signal applied to terminals 52 and I! includes the frequencies to which the contactors 'Ii and I2 are responsive, these contactors will intermittently A bridging condenser '34 across the contactor l2, resistor I5 for limiting the charging current and resistor l5 for discharging the condenser I3 are provided as in the first section or" the system. Energy for the charging circuit is obtained from the winding TI of the relay 7% which is in the utput circuit of the triode is in the same manner that the relay 55 is connected to the triode 52. It will be apparent, therefore, that when a predetermined voltage is built u on condenser l3 the relay l3 will be energized causing the contact 86 to close so that a signalling device such as the cell 8! In the system just described it is apparent that noise or voice signals including the frequencies to which contactors I! and I2 respond will be entirely inefiective to produce a voltage on condenser 13 until the first section of the system has operated so that the windings E9 and HE are connected to the signal source. Therefore, in order for the system to be accidentally tripped by noise, the noise must include the frequencies of contactors 5i and 5! for a suificiently long time to trip the first section and then contain the frequencies of contactors Ii and I2 for a sufficient time to trip the second section.

In Fig. 3 there is illustrated a modified selective responsive system which includes a plurality of sections as in Figs. 1. and 2 and in which the energizing potential is transferred from the first section to the second as in Fig. 1 and the calling signal is also transferred from the first section to the second as in the system of Fig. 2. In this system contactors It?) and till include windings I 02 and I 83 which are connected in parallel across the terminals I0 and i i of the signal source by the contacts Hi l and I5 of relay 953. The contactors IE0 and Id! are respectively bridged by condenser I06 and resistor III! in series and condenser I88 and resistor IIlil in series. A charging path is provided for condenser I IE] through the contactors I66 and Ifil and through resistor III between a +8 potential and ground. By providing condensers across each contactor, an increment of charge is provided on condenser Hi3 each time either contactor is operated. The resistors I01 and E39 limit the current through the contactors I89 and IEII when the condensers tilt and I98 are discharged thereby. Resistor IE2 is provided for discharging the condenser iiil as in the manner described above. The voltage across condenser I It is applied to the triode i 53, the condenser being connected to the grid il and through the winding i I5 of relay 99 to Cathode I It. The plate i ll of the relay is connected through resistor I I8 to the +3 potential so that when the voltage of condenser I reaches a predetermined value, the triode H3 will conduct to energize the winding I I5 of relay 92 This will cause movable contact I04 to move away from fixed contact 5 05 disconnecting the windings Edi and 83 from the signal source.

Operation of the relay 99 causes the movable contact 586 to engage fixed contact II!) to connect the windings I26 and IEI of the contactors E22 and 23 of the second section of the system in parallel to the signal source. The contactors and I23 are arranged to charge the condenser IE5 of the relay. As in the first section of system the contactors are bridged by condensers and resistors in series and the main condenser i24 being bridged by a discharging resistor I25. The voltage across condenser I24 is applied to triode I28 being applied to the grid I29 and through the winding i2? of relay I28 to cathode i311 of the triode. The plate I31 of the triode is connected through resistor I32 to the +B potential so that when the voltage on condenser I24 reaches a predetermined value the winding I21 of relay iZt is energized to cause the movable contact 33 thereof to be moved away from fixed contact Isa into engagement with fixed contact I35. These contacts may be used to provide any desired switching operation as to connect a loudspeaker as illustrated in Fig. 1 or to energize a signalling bell as illustrated in Fig. 2.

The system of Fig. 3 includes the advantages of both the systems of Fig. l and Fig. 2 in that neither energy nor the calling signal is applied to the second section of the system until the first section operates. In this system accidental trip is not possible until the proper frequencies are applied to the first section for a sufiiciently long time to energize it and then the proper frequencies for the second section are applied for a sufficient length of time, since the second section is not affected by noise until the first section has been operated.

In Fig. 4 a still further system is illustrated for providing a control voltage in response to calling signals of predetermined frequencies. In this system the electromechanical contactors are all simultaneously connected to the signal source and energizing potential is applied successively to the various sections as each section responds to the calling signal. The first section includes contactors I49 and I4! adapted to charge condenser I42 from the +B potential. When a signal including the frequencies to which contactors I40 and HI respond is applied, the condenser I42 will be charged causing the triode M3 to conduct. A potential is thereby built up on the resistor M in the cathode circuit of the triode I43 which potential provides energy for the second section. The second section includes contactors I45 and H6 which charge condenser I41 from the potential across resistor I44 when a signal including the frequencies to which contactors 8'5 and I46 respond is applied thereto. The voltage on condenser i li is used to trigger triode I48 so that a voltage is built up across the resistor I49 in the cathode circuit thereof. This voltage is transferred by contactors I58 and II to condenser I52 in the same manner resulting in a voltage on resistor I53 in the cathode circuit of the triode I 54. A fourth section is illustrated including contactors I55 and E56 which transfer the potential from resistor I53 to condenser I51 which causes operation of the triode I58 to thereby energize the relay 559. It is apparent that any number of sections can be provided as is necessary to provide the number of difierent calls desired.

The system of Fig. 4, in addition to providing a large number of calls, also provides highly effective discrimination against noise. Although the signal is simultaneously applied to all of the contactors, energy for the charging circuit of each section is not available until the preceding section has been tripped. This system is generally "similar to the system of Fig. l in this respect except that a partial voltage built up on the condenser M2 on the first section will not provide a reduced charging voltage for the second section as the charging voltage is obtained across resistor hit which will not provide a potential until the tube M3 conducts. Therefore, each section is entirely de-energized until the preceding section has been tripped and, for example, noise of the frequencies to which contactors i 35 and 14.6 respond will be ineffective to charge the condenser Ml until'tube I43 is rendered conductive by the charge on condenser M2. Similar operation will take place in the various sections and high discrimination against noise will be provided.

The provision of a plurality of pairs of signals in'sequence provides a much larger number of calls when usingelectromechanical contactors responsive to a given number of different frequencies. This, therefore, provides a system which "would be applicable in networks having a very large number of stations. Although the system may be used when the frequencies for all of the contactors are transmitted simultaneously improved operation has resulted when the pairs of frequencies for the various sections are transmitted in sequence. When all of the signals are sent simultaneously the number of possible calls is reduced as the same frequencies cannot be use'din-any two contactors. Also discrimination against'noise is better when the signals are transmitted in sequence. In the systems shown in .Figs.i2 and 3 the various sections of the system are completely isolated with the calling signal being applied to only one section at-a time. This would tend to increase the discrimination of the system against noise as the second section is dis connected from the signal source over a large part of the time and voltage would not be built up on the charging condenser thereof by random noise. As in the systems of Figs. 1, 3 and 4 operating voltage for the second section is notapplied thereto until th first Section is operated,

random noise will not be effective to produce a charge on the condenser of the second section. It is also to be pointed out that each section of the systems only remains tripped for a limited il (l VII time so that the required combinations of fre quencles must be applied within a short time interval to operate the systems.

The selective callin systems as described require relatively simple equipment which is not critical of adjustment. A two-section unit requires only a single tube, a twin triode being suitable for the systems of Figs. 2 and 3 or for a twosection system in accordance with Fig. 4. Such a system can be installed in a very small space as available in a mobile communication receiver.

While I have described certain embodiments of my invention which are illustrative thereof, it is obvious that various changes and modifications can be made therein without departing from the intended scope of the invention as defined in the appended claims.

I claim:

1. A system for providing selective operation in response to a calling signal including a plurality of predetermined frequencies, said system including a plurality of pairs of electromechanical contactors individually responsive to said predetermined irequencies, a plurality of impedance elements individually associated with said pairs of contactors, a source of potential, a first circuit including said source of potential, said coutactors of one of said pairs and said associated impedance element for producing a voltage across said impedance element in response to a calling signal including frequencies to which said contactors of said one pair are responsive, and at least one additional circuit including another pair of Said contactors and the associated impedance element, said additional circuit providing a voltage across said impedance element thereof when a voltage of a predetermined value exists across said impedance element of the preceding circuit, and in response to a calling signal including the frequencies to which the contactors of said additional circuit are responsive, each of said circuits providing a voltage across th impedance element thereof which exceeds said predetermined value for an interval of time after the frequencies to which the particular circuit responds have been applied thereto.

2. A system for providing selective operation in response to a signal including a plurality of predetermined frequencies, said system including in combination, a plurality of pairs of frequency selective devices each of which includes means for providing a conductive path in respons to one of said predetermined frequencies, a plurality of impedance elements individually associated with said pairs of frequency-selective devices, a

source of potential, a first circuit including said source of potential, said devices of one of said pairs and said associated impedance element for producing a voltage across said impedance element in response to a signal including frequencies to which said devices or" said one pair are responsive, and at least one additional circuit coupled to said impedance element of said first circuit and including another pair of said fre quency selective devices and the associated impedance element, said additional circuit providing a voltage across said impedance element thereof when a voltage exists across said impedance element of the preceding circuit, and in response to .a signal including th frequencies to which the devices of said additional circuit are responsive.

3. A selective system including in combination, first and second pairs of frequency responsive devices each one of which includes means forming .a'conductive path in response to a .signalof a different frequency, a source of signals, a source of potential, first and second condensers, a relay including a winding and switch m ans arranged to normally connect said first pair of devices to said signal source to disconnect said first pair of devices and connect said second pair of devices to said signal source when said relay is operated, a circuit including said first pair of devices and said first condenser for charging said condenser when a calling signal including the freqeuncies to which said devices of said first pair are responive is applied thereto, a second circuit including said first condenser and said winding of said relay for operating said relay when said condenser is charged to a predetermined potential, and a third circuit incluuding said winding of said relay, said second pair of devices'and said. second condenser for charging said second condenser when said winding is energized and in response to a calling signal including the frequencies to which said devices of said second pair are responsive.

4. A system for providing selective operation in response to a Signal including a plurality of predetermined frequencies, said system including in combination a plurality of frequency responsive sections, each of said sections including input terminals, a pair of frequency responsive devices, and an electron discharge valve having an input circuit including a condenser and an output circuit including an impedance element, said pair of frequency responsive devices and said condenser of each of said sections being connected in a series circuit across said input terminals thereof, each of said frequency responsive devices including means forming a conducting path therethrough in response to one of said predetermined frequencies, a source of potential, and a circuit for connecting said frequency responsive devices to a source of signals, one of said frequency responsive sections having the input terminals thereof connected to said source of potential for charging the condenser of said one section in response to a signal including the frequencies to which said pair of devices of said one section respond, said condenser being effective to render said electron discharge valve conductive so that relatively large current fiows through said impedance element of said one section providing a relatively large voltage thereacross, said additional sections being connected to said first section in cascade with the input terminals of each section connected to the impedance element of the preceding section so that the voltage is transferred to said impedance elements of each section in turn in response to a signal including the frequencies to which the frequency responsive devices of each section are responsive.

5. A selective responsive system including in combination, first and second pairs of frequency selective devices each one of which includes means forming a conductive path in response to a signal of a particular frequency, a source of potential, first and second condensers, an electron discharge valve having input and output electrodes, impedance means, a first circuit including said first pair of devices, said source of potential, said first condenser, said electron discharge valve and said impedance means, said first circuit charging said first condenser when a calling signal including the frequencies to which said devices of said first pair are responsive is applied thereto, said first condenser being connected to said input electrodes of said valve and said impedance means being connected to said output electrodes thereof so that when said first condenser is charged to a predetermined value said valve conducts and a voltage is developed across said impedance means, and a second circuit including said impedance means, said second pair of devices, and said second condenser, said second circuit charging said second condenser from said potential across said impedance means in response to calling signals of the frequencies to which said devices of said second pair are responsive.

6. A system for providing selective operation in response to signals including particular predetermined frequencies comprising, a source of potential, first and second pairs of electromechanical devices each of which includes means forming a conducting path in response to one of said predetermined frequencies, first and second condensers, first and second electron discharge valves having input and output electrodes, an impedance element, a relay, a first circuit including said source of potential, said first pair of devices, and said first condenser for charging said condenser from said source of potential in response to a signal including the frequencies to which said first pair of devices respond, said first condenser being connected to said input electrodes of said valve and said impedance element being connected to said output electrodes thereof so that said condenser renders said valve conducting and a relatively large voltage is devel-- oped across said impedance element, and a second circuit including said impedance element, said second pair of electromechanical devices and said second condenser for charging said second condenser from the voltage across said impedance element in response to a signal including the frequencies to which said second pair of devices respond, said second condenser being connected to said input electrodes of said second electron discharge valve and said relay being connected to said output electrodes thereof so that said second condenser renders said second electron discharge valve conducting and said relay is energized thereby.

'7. A system for providing selective operation in response to signals including particular predetermined frequencies comprising, a source of potential, first and second pairs of electromechanical devices with each device including means forming a conducting path in response to one of said predetermined frequencies, first and second electron di charge valves having input and output electrodes, an impedance element, a first circuit including said source of potential, said first pair of devices, and said input electrodes of said first electron discharge valve for rendering said valve conductive in response to a signal including the frequencies to which said first pair of devices respond, said impedance element being connected to said output electrodes of said first electron discharge valve so that a relatively large voltage is developed across said impedance element, and a second circuit including said impedance element, said second pair of devices and said input electrodes of said second electron discharge valve for rendering said second electron discharge valve conductive in response to a signal including the frequencies to which said second pair of devices respond.

8. A selective responsive system including first and second pairs of electromechanical contactors each one of which is responsive to a calling signal of a different frequency, a source of calling signals, first and second condensers, a relay including a winding and switch means arranged ii to normally connect said first pair of'contactors to said signal source and to disconnectsaid first pair of contactors and connect said second pair oi'contactors to said signal source when said reiay isoperated, a circuit including said first pair of contactors, and said first condenser for charging said condenser when a calling signal including the frequencies to. which said contactors of said first pair are responsive is applied thereto, a second circuit including said first condenser and said Winding of said relay for operating said relay when said condenser is charged to a predetermined potential, and a third circuit including said second pair of contactors and said second condenser for. charging said second condenser in response to a calling signal including the frequencies to which said oontactors of said second pair are responsive.

9. A selective responsive system including first and second pairs of electromechanical contactors each one of which is responsive to a calling signal of a different frequency, a source of calling signals, a source of potential, first and second condensers, a relay including a winding and switch means arranged to normally connect said first pair of contactors to said signal source and to disconnect said first pair of contactors and connect said second pair of contactors to said signal source when said relay is operated, a circuit including said first pair of contactors and said first condenser for charging said condenser when a calling signal including the frequencies to which said contactors of said first pair are responsive is applied thereto, a second circuit including said first condenser and said winding of said relay for operating said relay when said condenser is charged to a predetermined potential, and a third circuit including said winding of said relay, said second pair of contactors and said second condenser for charging said second condenser when said Winding is energized and in response to a calling signal including the frequencies to which said contactors of said second pair are responsive.

10. A selective responsive system comprising first and second pairs of electromechanical contactors each one of which is responsive to a calling signal of a different frequency, a source of calling signals, a source of potential, first and second condensers, first and second electron discharge valves, impedance means, a relay, a first circuit including said first pair of contactors, said source of potential, said irst condenser, said first electron discharge valve and said impedance means, said first circuit charging said first condenser when a calling signal including the frequencies to which said contactors of said first pair are responsive is applied thereto, said first condenser being connected to the input electrodes of said first valve and said impedance means being connected to the output electrodes thereof so that when said first condenser is charged to a predetermined value said valve conducts and a voltage is developed across said impedance means, and a second circuit including said impedance means, said second pair of contactors, said second condenser, said second electron discharge valve and said relay, said second circuit charging said second condenser from said potential on said. impedance means in response to calling signals of the frequencies to which said contactors of said second pair are responsive, said second condenser being connected to the input electrodes of said second electron discharge valve and said relay being connected to the out- I put electrodes thereof so that when said second i2 condenser is charged to a predetermined value said relay is energized.

11. A system for providing selective operation in response to a calling signal including a plurality of predetermined frequencies, said system comprising a plurality of frequency responsive sections, each of said sections including input terminals, a pair of electromechanical contactors, and an electron discharge valve having an input circuit including a condenser and an output circuit including an impedance element, said pair of electromechanical contactors and said condenser of each of said sections being connected in a series circuit across said input terminals thereof, said electromechanical contactors being individually responsive. to said predetermined frequencies, a source of potential, and a circuit for connecting said electromechanical contactors to a source of calling signals, one of said frequency responsive sections having the input terminals thereof connected to said source of potential for charging the condenser of said one section in response to a signal including the frequencies to which said pair of contactors of said one section respond, said condenser being effective to render said electron discharge valve conductive so that relatively large current flows through said impedance element of said one section providing a relatively large voltage thereacross, said additional sections being connected to said first section in cascade with the input terminals of each section connected to the impedance element of the preceding section so that the voltage is transferred to said impedance elements of each section in turn in response to a calling signal including the frequencies to which the contactors of each section are responsive.

12. A system for providing selective operation in response to calling signals including particular predetermined frequencies comprising, a source of potential, first and second pairs of electromechanical contactors with the contactors being individually responsive to said predetermined frequencies, first and second condensers, first and second electron discharge valves having input and output electrodes, an impedance element, a relay, a first circuit including said source of potential, said first pair of contactors, and said first condenser for charging said condenser from said source of potential in response to a calling signal including the frequencies to Which said first pair of ccntactors respond, said first condenser being connected to said input electrodes of said valve and said impedance element being connected to said output electrodes thereof so that said condenser renders said valve conducting and a relatively large voltage is developed across said impedance element, and a second circuit including said impedance element, said second pair of contactcrs and said second condenser for charging said second condenser from the voltage-across said impedance element in response to a calling signal including the frequencies to which said second pair of contactors respond, said second condenser being connected to said input electrodes of said second electron discharge valve and said relay being connected to said output electrodes thereof so that said second condenser renders said second electron discharge valve conducting and said relay is energized thereby.

13. A system for providing saective operation in response to calling signals including particular predetermined frequencies comprising, a source of potential, first and second pairs of electromechanical contactors with the contactors being individually responsive to said predetermined frequencies, first and second electron discharge valves having input and output electrodes, an impedance element, a relay, a first circuit including said source of potential, said first pair of contactors, and said input electrodes of said first electron discharge valve for rendering said valve conductive in response to a calling signal including the frequencies to which said first pair of contactors respond, said impedance element being connected to said output electrodes of said first electron discharge valve so that a relatively large voltage is developed across said impedance element, and. a second circuit including said impedance element, said second pair of contactors and said input electrodes of said second electron discharge valve for rendering said second electron discharge valve conductive in response to a calling signal including the frequencies to which 14 said second pair of contactors respond, said relay being connected to said output electrodes of said second electron discharge valve and being energized thereby.

DANIEL E. NOBLE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,173,154 Bernard Sept. 19, 1939 2,457,149 Herbst Dec. 28, 1948 FOREIGN PATENTS Number Country Date 172,321 Great Britain Nov. 2, 1922 261,384 Great Britain Oct. 6, 1927 

